http://radiohead1.tripod.com/songs/bsides/howcanyoubesure.htm
Well according to this source, her name is Dianne Swann, formally of the band The Julie Dolphine, but now in a band called The Bads. I kinda felt that those were not just Ed's vocals alongside Thom's.
This song is simply beautiful. Thom's vocal <333. One of my favourite b-sides